Patients and Methods
Details of the design of the multi-element study of bone health and body
composition have been published.⁴ From a cohort of children and
adolescents who were diagnosed with ALL more than a decade earlier at
McMaster Children’s Hospital (MCH) a study sample was constructed (Fig
1). Treatment had been administered on protocols of the Dana Farber
Cancer Institute Childhood ALL Consortium⁷ of which MCH had been a
member since 1985.
The current report focuses on bone geometry, density and architecture as
assessed by pQCT performed with a Stratec XCT 2000 instrument (Stratec
Medezintechnik, Pforzheim, Germany) with the Stratec Analysis Software
v6.0 (Orthometrix Inc, White Plains, NY, USA). Measurements were made by
a single experienced operator at the 4 % sites of the length of the
radius and tibia in the non-dominant limbs as well as at the 20% site
of the radius and the 38% and 66% sites of the tibia, all with 0.2 mm
voxel size and 2.2 mm slice thickness. The 4% (metaphyseal) sites
represent trabecular bone and the proximal (diaphyseal) sites cortical
bone.⁵ In order to emphasize metrics of high clinical importance and
relevance to bone biology and biomechanics, including bone strength, as
well as to allow comparisons with published reference values, analyses
focused on 4 metrics at the metaphyseal sites – Total bone mineral
content (BMC), Total vBMD, Trabecular BMC and Trabecular vBMD – and 6
metrics at the diaphyseal sites – Total BMC, Total vBMD, Cortical BMC,
Cortical vBMD, Cortical Thickness and Polar Strength-Strain Index (SSI);
a total of 10 selected from the many metrics provided by the Stratec
software.
In addition to the standard volumetric density and geometry endpoints
derived from the Stratec analysis software, measures of apparent
trabecular architecture captured at the 4% metaphyseal sites were
derived from the pQCT images. Specifically, the 4% metaphyseal images
were post-processed with custom software developed by one of the
authors.⁸ This builds on the work of CLG and colleagues, beginning 25
years ago.9-12 As reported,8 the
software uses edge detection and thresholding steps to segment the
trabecular bone and to quantify its structure. Fig 2 illustrates some of
the image processing steps. The connectivity of the trabecular network
is based on strut analysis in which the network is considered to consist
of one-dimensional struts (Fig 2A).9 A junction
between three or more struts is defined as a node. A strut which is
connected at one end but is free at the other is called a free end.
Struts representing trabeculae running perpendicular to the image appear
as points and are deemed to be isolated points. The connectivity of the
bone architecture can also be assessed by quantifying the appearance of
holes in the network due to loss of trabecular elements (Fig 2B). As
more elements are lost, the average hole size reflects the increased
mean distance between trabecular struts.11 Overall,
from the 78 metrics available from post-processing, 7 were selected with
respect to apparent trabecular bone architecture– Trabecular Thickness
(mm), Trabecular Number (1/mm), Average Hole Size
(mm2), Maximum Hole Size (mm2),
Number of Free Ends per Unit Area (Free End Density –
mm-2), Number of Isolated Points per Unit Area
(Isolated Points Density – mm-2) and Connectivity
Index (Number of Nodes per Unit Connected Network). A bone with high
connectivity, and so strength (resistance to bending, torsion and
compression), has a large number of nodes with few free ends and
isolated points as well as small marrow pores (hole sizes). The custom
program allows greater sensitivity to detect changes associated with
perturbed bone metabolism that we have described in children with
ALL.13
Of note, pQCT images can be acquired in a short period of time (10
minutes) with low radiation exposure (approximately
5µSv),14 equivalent to about 2 days of natural
radiation. Comparisons were made with published data on healthy subjects
data from relevant age groups using Stratec XCT 2000 and 3000
instruments.5,15-23 Comparative values for the 4%
radius site are available from 4 publications,15, 18,
19, 22 and there are 4 reports on the 4% tibia
site.16,19,21,23 Fewer were found for the diaphyseal
sites – 20% radius,19 38%
tibia,17,20,21 and 66% tibia.16,23Information from these sources were provided in Supplementary Tables 1
and 2. There is no “gold standard” set of reference values for pQCT in
children and adolescents.24 However, reference values
for comparison at the 4% radius22 and the 4% and
38% tibia25 are available from a sample of more than
220 healthy Caucasian subjects, allowing the calculation of Z scores at
these sites. The normative values for those aged 20-40 years are taken
as equivalent to those in healthy 19 year old males and
females.26 These data were obtained using Stratec
equipment and software very similar to those used in the current study.
In light of the differences in body composition in this cohort according
to sex,27 the pQCT metrics were also examined
separately in males and females. Data from pQCT are presented as means
and standard deviations or as medians and ranges when appropriate.
Subgroup analyses were undertaken on the 14 subjects who had received a
bisphosphonate during therapy to correct osteoporosis, defined as an LS
BMD Z score < -2.0.28,29 Parametric data
were compared with 2 tailed t tests and non-parametric data by
chi-squared tests. Correlations were measured with Pearson’s correlation
coefficient for parametric data and with Spearman’s rho for
non-parametric data. Comparisons of groups who did and did not receive a
bisphosphonate were performed by Fisher’s exact test. All analyses were
performed with SPSS, version 24.
This study was approved by the Hamilton Integrated Research Ethics Board
(project 10-508-5) which represents Hamilton Health Sciences, McMaster
University and St. Joseph’s Healthcare Hamilton. Informed consent was
obtained from each participant.